The invention relates to a gas-proof assembly composed of a bipolar plate and a membrane-electrode unit of polymer electrolyte membrane (PEM) fuel cells, a method for its production and the application of the assembly in a serial fuel cell stack.
A PEM fuel cell consists of two current collector plates, two porous, possibly catalyzed gas diffusion layers and a catalyzed or non-catalyzed ion exchange membrane which is arranged between these layers. No uniform technical terminology has been established yet with regard to the assembly components; occasionally the gas diffusion layers are described as electrodes, and occasionally the catalyst layers that are applied onto the membrane are also described as electrodes. The current collector plates are typically equipped with devices for feeding and distributing the reactants, so-called gas distribution structures. Since the electric voltage of a single cell is much too low for practical applications, a plurality of such cells have to be connected in series. In the resulting fuel cell pile or stack, the current derivation plates that coincide are termed bipolar plates. A bipolar plate, with one of its surfaces, is electrically connected to the anode of a cell of the stack, while the opposite surface is in contact with the cathode of the neighboring cell. The function of the bipolar plates is, on the one hand, to conduct the current through the stack, and on the other hand to separate the reaction gases. Furthermore, they are also usually equipped with gas distribution structures, such as a channel system, for better distribution of the reaction gases in the anode zone and the cathode zone.
By feeding the typical reaction gas hydrogen to the anode side of the fuel cell, cations are generated in the catalyst layer that is in direct contact with the ion exchange membrane at the anode side, and at the same time electrons are passed on to the anode side electron conducting layers. The oxidizing agent that is typically used is oxygen (or air), which is fed to the cathode side of the cell. The reaction gas oxygen is reduced by absorbing both the hydrogen ions that have diffused through the ion exchange membrane and the electrons that are fed from the anode to the cathode via an external circuit. This reaction also takes place in a catalyst layer that is in contact with the membrane on the cathode side. In preferred applications, the oxygen concentration in the air is sufficient. The reaction product is water. Reaction enthalpy is released in the forms of electric energy and of waste heat. The assembly of the membrane and the gas diffusion layers or electrodes, including the respective catalyst layers, is termed the membrane electrode assembly (MEA) in the following. As mentioned above, it has not yet been uniformly established in the literature whether the xe2x80x9celectrodesxe2x80x9d include portions of the gas diffusion layer or whether only the catalyst forms the electrodes. In the following, this will be pointed out should a differentiation be required for better understanding.
A considerable problem in the design of fuel cell stacks is the permanent seal of the anode zone. Due to the high avidity of hydrogen, this feature is required not only for achieving good utilization of energy, but also for safety reasons. If air or oxygen is used at excess pressure, the cathode zone must be sealed as well.
Many sealing systems require considerable pressure on the peripheral sealing edge in order to achieve the necessary sealing effect. This means that the clamping plates have to have larger dimensions and thus make the entire stack heavier, which is disadvantageous for mobile applications. The use of clamping elements at the frame generates additional weight due to metal parts with relatively thick walls. Mutual adjustment of the thicknesses of the electrodes and the bipolar plates and the thickness of the seal is extremely difficult because both the electrodes and the seal require suitable pressure, but have different degrees of elasticity. Tolerable thickness deviations are very small. This requirement leads to complex manufacturing procedures, which are very cost-intensive. The usage of different materials for the bipolar plates and the seals also causes the risk that leaks occur upon start-up because the different materials have different degrees of expansion when warming up. If elastomer seals are employed, thin membranes frequently rupture at the clamping step due to the change in length of the elastomer (e.g., silicone).
One method for sealing the gas chambers of PEM fuel cells consists of the production of seals with elastomer materials and the arrangement of these seals between the polymer electrolyte membrane and the bipolar plates, which are made of gas-proof graphite materials. To accomplish this, the seal is placed in slots that have been manufactured in a complicated process and that are provided for, exclusively for this purpose, in a carbon fiber paper which serves as a gas diffusion layer. Such an application can be found, for example, in U.S. Pat. No. 5,284,718.
The seal can also be formed by an elevation that is integrated in the bipolar plate and is formed by a stamping process. In this case, however, the bipolar plates will have to be made of an elastic, plastically deformable and gas-tight material, e.g. of graphite foils. Also, the seal requires, in this case, considerable pressure for achieving the sealing effect, which must be exercised by the clamping plates. Such a method is described e.g. in DE-OS 195 42 475 A1.
Another sealing method is presented in DE-PS 44 42 285 C1, where the negative polar plate, the membrane, the positive polar plate and two seals are clamped with each other at the periphery by a frame element in a gas-tight and electrically insulating manner. The frame element, which consists of metal, can be part of a polar plate and has a U cross-section. By expanding this U section element during assembly, the necessary pressing forces are generated.
It is also possible to manufacture a unit from a seal layer and the ion exchange membrane, as shown in EP-PS 0 690 519 A1. The seal layer, which consists of porous polytetrafluoroethylene, is applied to the membrane on both sides and surrounds the part of the membrane that is coated with the catalyst like a frame.
From JP 09-289029 we know of glued cells and also glued stacks, which are manufactured individually, stacked and then glued together to a stack. The figures of this publication show the glue-assembly with a square frame 18, shown in its top view in FIG. 4, i.e. a pre-formed glued insert. A comparable arrangement is also shown in WO 94/25995. It includes a frame element that is glued in by utilizing a silicon sealant. Due to manufacturing tolerances, but also due to their purpose, i.e. easy insertion, such pre-fabricated frames, however, do not completely fill the gap because the seal must be cut out larger than the gas diffusion layer, and the problem of the formation of a gap arises, especially on the outer side of the electrically conductive gas diffusion layer, between this layer and the gluing frame. In the area of this gap, the membrane rests neither against the seal nor against the gas diffusion layer and is therefore without support. Due to considerable expansion and shrinkage of the membrane in connection with environmental factors, especially humidity, these gaps are frequently the starting point for cracks in the membrane, which represent a destruction of the cell. There is especially increased risk for the formation of cracks in the membrane in case of heavily swelling membranes and very thin membranes, such as membranes made of sulfonated polyetherketone.
The invention is intended to prevent the formation of such a gap safely. This is accomplished by filling the volume zone, which surrounds the gas diffusion layer at the outside, all the way to its defining surfaces with an adhesive that has cured there, without gaps and in a gas-tight manner. In accordance with a preferred version, the adhesive even penetrates for a little length into the diffusion layer, and in accordance with another very important special version, such gap-free glued seals are incorporated not only on the outer circumference of the gas diffusion layer between the bipolar plate and the membrane, but also between these components where gas conducts are running through.
Additional preferred versions of the invention can be found in the respective sub-claims.
The invention is explained more in detail in the following, while referencing the drawings, by presenting an exemplary embodiment and intermediate stages of its manufacturing process. In the drawings show: